1. Field of the Invention
The present invention relates to a vehicle and a method for operating a vehicle to reduce exhaust emissions.
2. Background Art
Vehicles have long employed the use of catalysts to reduce engine exhaust emissions. Generally, a catalyst facilitates a reaction between oxygen and unburned hydrocarbons, carbon monoxide, and various nitrogen/oxygen compounds, thereby producing carbon dioxide, nitrogen and water. One limitation of the use of catalysts is that they tend to be ineffective below a certain activation temperature. Thus, below the activation temperature, unburned hydrocarbons, carbon monoxide and nitrogen/oxygen compounds can be exhausted from the vehicle and introduced into the atmosphere.
In order to address this problem, a number of systems have been developed to reduce vehicle emissions prior to a catalyst reaching its activation temperature. One such system is described in U.S. Pat. No. 6,327,852 issued to Hirose on Dec. 11, 2001. Hirose describes the use of a catalyst in conjunction with an adsorbent as part of an exhaust gas emission control apparatus. The catalyst is used to reduce emissions after it has reached its activation temperature. Before the catalyst has reached its activation temperature, the adsorbent is used to adsorb unburned fuel to prevent its release into the atmosphere. One limitation of the apparatus described in Hirose, is that it relies on a separate device—i.e., the adsorbent—to augment the catalyst as part of an emission control system.
Another method of controlling exhaust emissions is to provide a mechanism for quickly heating the catalyst to reduce the amount of time it takes the catalyst to reach its activation temperature. One way to quickly heat the catalyst is to retard the spark timing of the engine. This causes the catalyst to be heated more quickly, but results in a loss of engine power. Thus, if a driver demands an increase in engine power, the spark retard must be reduced, which causes the catalyst to be heated more slowly, and results in more exhaust emissions being introduced into the atmosphere.
The development of hybrid electric vehicles (HEV's) has provided a way to reduce overall engine exhaust emissions, by reducing the amount of time an engine is running. One factor that limits the reduction of exhaust emissions in an HEV is that the engine will typically be stopped and started many more times than in a conventional vehicle. Thus, there is the potential for a great deal of exhaust emissions to be released into the atmosphere during the many times an engine in an HEV is started and operated prior to the catalyst reaching its activation temperature.
Therefore, a need exists for a vehicle and method for operating a vehicle to reduce the amount of exhaust emissions released into the atmosphere following engine start-up.